write more than a byte, by SPI !UERGENT please!

henzelince · Joined: 14 May 2007 Posts: 3

I want to write 24 bits by SPI hardware module.

If I write

spi_write(0b01010101);// i loose time
spi_write(0b10101010);//i loose time
spi_write(0b01010101);

there are some latency time between each spi_write.

If i write

spi_write(0b010101011010101001010101);// that write only a byte.

How can I do for write these 24 bits with any timeout, with the hardware SPI only ??

Ttelmah · Guest

At the end of the day, you can't. However the time loss, should be tiny. Basically, you can only load a 'new' value into the SPI output register, once the last value has been sent. (The transmit register is not 'double buffered' like the receive). Hence you need to check either the SSPIF, or the BF bit, to know that the eight clocks have been sent. Once this is seen, you can load another byte. The total time delay, should be reducable to only about 4 instruction tims, but is unavoidable. You could try coding this yourself, to see if this is more efficient than the CCS code (which probably checks for other things like overrun etc.).
Something like (for a 18 chip):

henzelince · Joined: 14 May 2007 Posts: 3

thx !!!

I'll try this solution !

henzelince · Joined: 14 May 2007 Posts: 3

Ok that works !

For another user:
#byte SSPSTAT=0xFC7 // not 0xFC8

Ttelmah · Guest

Typical of my sort of fault. As you possibly guessed, I typed this 'untested', straight into the thread. Though I try to scan for obvious errors, things like this 'creep by'.
Glad it worked.

Best Wishes